Field boots (lacing at the ankle), or 'real' tall boots (no lacing anywhere)? The former is easier to break in properly than the latter IME.
The biggest thing is to get boots that fit you properly in the first place. I know this is difficult for some people (me! me! me!) but ill-fitting boots, that are too tight somewhere or too sloppy somewhere or the wrong height, or just poorly made, are never going to come really right for ya.
What I was taught to do, and thus have always done, and it works good for me, is to discreetly rub a *tiny little whiff* of neatsfoot oil on the inside of the front of the ankle, where you need the crease to form, then work it back and forth with your hands (the boot isn't on your foot yet) to get the crease started properly, and then ONLY WEAR THE BOOTS WHEN YOU ARE ON THE HORSE, PERIOD until they are well and truly thoroughly set in their ways. This means get the horse tacked up and lead it to the mounting block, then sit on the mounting block putting your boots on and immediately get on; and when you dismount, do it right next to your bootjack and barn boots, and take off the tall boots right away. If you walk around in them at all, you get the accordion-y saggy baggy creases started, and you will never be able to get rid of them then.
Store them with, whatchacallit, the thingies you put in the top to expand them like your calves would, those things in them, and with the foot 'formed' into a heels-down position with the correct crease.
For good-quality boots made with significant thickness of leather, I've known people who do the first few initial rides wearing damp socks inside the boots, to help the leather start molding into the right shape. I've never been able to afford boots you needed to do that with
but I would imagine you need to be a little careful about making sure they air out and dry well inside so's to avoid mildew.
Good luck, have fun,
Pat